In present day industrial scale grinding systems, fine grinding of minerals or other substances is accomplished in large cylindrical rotating grinding chambers referred to as ball mills, rod mills, pebble mills, or autogenous mills which employ steel balls, steel rods, pebbles, or large pieces of ore respectively as the grinding media and from which the name of the mill is derived. The new ore, or other mineral or substance to be ground, is directed into the grinding chamber at one end of the rotating cylinder, and the ground ore, commonly referred to as fines, is removed at the other end. The mills are charged with the grinding media, i.e., steel balls, rods, etc., as needed. The mills are of the overflow or grate discharge types employing either dry or wet grinding. Dry grinding is used primarily for cement manufacture and wet grinding primarily in metal ore applications. In wet grinding, a slurry is made of the ore and a fluid, usually water. The efficiency of wet grinding depends upon the slurry consistency, grinding media, and the mill inventory consisting of the ore in the grinding chamber. Dry grinding efficiency depends only upon the mill inventory and grinding media.
Control of the slurry consistency has been the subject of considerable research and successful methods have been found to control this parameter. Accurate control of mill inventory, however, has eluded investigators. Mill inventory effects ore throughput of the grinding system in the following way: if the ore to grinding media ratio is too small, the grinding media grinds against itself and the mill liners causing excessive wear; if the ratio is too large, the ore and water slurry cushions the impact of the falling grinding media and restricts its movement. FIG. 1 illustrates the relationship of ore throughput to the ore weight in the mill, remembering that the weight of all components in the mill grinding chamber, except ore are essentially constants, i.e., water, or are changing at known rates, i.e., the wearout rate of the grinding media and thus the associated weight change is known. See for example, Inventory Control of Grinding Mills Using Bearing Pressure Measurement, D. J. Oswald and J. G. Ziegler, Transactions, Society of Mining Engineers, AIME, Vol. 254, Sept., 1973, pp. 201-205.
In the past, large amount of research has been expended on mill inventory control using indirect variables such as the mill drive motor power draw, the sound eminating from the grinding chamber, and the circulating load. The first method above has not been successful inasmuch as there is a detectible change in power consumption only after the mill has reached a point of overload. Sound detection methods are not satisfactory because the sound produced in the grinding chamber is not closely related to the mill inventory. Circulating load (which refers to the insufficiently ground ore which egresses the grinding chamber, but is returned to the mill by separators or classifiers for regrinding) measurements are unsatisfactory because of difficulty in measuring the circulating load, and circulating load is not necessarily a proportional indicator of the mill inventory, especially if the mill has reached a point of plugging up.
Using these control methods, most mills operate at a point below the peak of maximum throughput vs. mill inventory. The difference between the average operating point and peak throughput has been estimated to be in the range of 5 to 20 percent below peak throughput.
Control of mill inventory by sensing the mass of the mill and content has been attempted before, see for example, U.S. Pat. No. 3,350,018, which uses pressure of the oil lubrication film of supporting mill journals to sense the weight. However, the method there described suffers from low sensitivity to mill inventory change, inaccuracies due to oil viscosity variation and temperature effects, and further, the system fails to take into account weight changes which are due to grinding media and mill liner wear. Additionally, the above system fails to account for the variable vertical forces applied to the mill by the drive means but which simulate inventory changes.
The low sensitivity problem, which affects all direct mass measurement methods, is due to the large ratio of mill and grinding media weight to ore weight, typically in the range of 25 to 1. If the inventory resolution required is plus or minus 2.5%, a mass resolution of plus or minus 0.1% is required. Recent advances in low drift electronics and load cells of the flat type have made measurements of this accuracy possible and relatively inexpensive. Flat load cells are also stiff enough to prevent mechanical misalignment problems. But still further, any direct mass measuring system must account for grinding media and mill liner wear, however, inasmuch as the rate of wear of the crushing medium and mill liners is approximately known, the relative substantial change in the total weight of the mill and mill inventory through the course of the day can be estimated fairly closely.
The exact curve of ore throughput versus weight of the ore in the mill is not known. However, it is known that the curve is unimodal and its general shape is as shown in FIG. 1. As can be seen by FIG. 1, there is a point of maximum efficiency where on either side of this point the mill is not operating at maximum efficiency. It is the determination of this point of maximum efficiency and the operation of the mill at that point thereafter to which this invention is directed.